Ferret is a full text search engine based on the popular Lucene Engine, which is originally written for Java. There is a great tutorial available here: http://www.railsenvy.com/2007/2/19/acts-as-ferret-tutorial

Assuming you have the plugin installed, acts_as_ferret in your vendor/plugins directory you'll need to install the ferret engine on your box. Use gems like so

gem install ferret

If you're running in dev mode, it should just work automatically, however in production, remember to start your ferret server in production mode with the following command! ./script/ferret_server start -e production